1. Field of the Invention
The present invention relates generally to a semiconductor device and a method of manufacturing the same, and more specifically to an SOC (System On Chip) technique for combining a DRAM device having a memory cell with a trench capacitor structure and a logic device on an SOI (Silicon On Insulator) substrate.
2. Description of the Related Art
With the developments in information communication technologies in recent years, in a semiconductor device which performs, e.g., image processing, the SOC technique for combining a DRAM device and a logic device in a single chip is desired in order to achieve high-speed data transfer between a memory and a logic circuit. It is one of the weighty SOC techniques to apply the SOI technique, which can realize a high-speed and low-power-consumption logic device, to the DRAM device.
FIG. 1 is a cross-sectional view schematically showing a memory cell formed when a DRAM having a memory cell with a trench capacitor structure is manufactured by an SOI technique. Specifically, FIG. 1 illustrates a conventional semiconductor device and a method of manufacturing the same. The memory cell is provided in an SOI substrate 11. The SOI substrate 11 includes a BOX (Buried Oxide) film 11-3 formed between a semiconductor region (substrate) 11-1 at a plate side and a semiconductor region (P-well region) 11-2 at an element region side. Specifically, the SOI substrate 11 is provided with a trench (deep trench) 12 with a depth to reach the substrate 11-1, extending from the P-well region 11-2 and passing through the BOX film 11-3. A capacitor (trench capacitor) 13 is formed within the trench 12. A cell transistor 14 for selecting the capacitor 13 is formed on the P-well region 11-2 in a state where one of source and drain regions 15, 16 of the transistor 14 is connected to one electrode of the cell capacitor 13.
However, with the above structure, since the P-well region 11-2 and the substrate 11-1 are electrically insulated by the BOX film 11-3, the P-well region 11-2 is in a floating state. This causes a problem that the channel potential of the cell transistor 14 cannot be controlled, and the operation becomes unstable.